1. Field of the Invention
The present invention relates to a sidestream infrared gas analyzer for continuously determining the concentration of the constituents of the respiratory gases of a patient. In particular, the present invention relates to a sidestream infrared gas analyzer employing an "optical funnel" to resize the optical aperture of a multi-channel detector without compromising signal strength, thereby allowing a smaller sample volume to be used for measurement.
2. Description of the Prior Art
It is frequently of critical importance to monitor the concentration of carbon dioxide (CO.sub.2) in the gases inspired and expired from a patient under anesthesia, for expired CO.sub.2 concentration is a reliable indicator of the carbon dioxide concentration in the arterial blood. In a clinical setting, monitoring expired CO.sub.2 prevents malfunctions in anesthesia rebreathing apparatus from going undetected and delivering excessive amounts of CO.sub.2 to the patient. Rebreathing of anesthetic gases is very cost effective and environmentally desirable, but accurate CO.sub.2 concentrations are difficult to maintain in the patient's circuit without a concentration monitor.
It is known that by directing infrared radiation through a sample of a gaseous mixture and measuring the incident radiation illuminating a detecting device, a measure of the infrared absorption of the gas can be obtained. Electrical signals produced by such a detecting device are indicative of the infrared absorption of the gas and can be processed to produce an output indicating the concentration of one or more of the constituents of the gas being analyzed. This type of gas analyzer operates on the principle that various gases exhibit substantially increased absorption characteristics at specific wavelengths in the infrared spectrum and that higher gas concentrations exhibit proportionally greater absorption.
Infrared respiratory gas analyzers for use in critical care applications come in two pneumatic configurations, namely, mainstream and sidestream. A mainstream analyzer is placed in the patient's respiratory circuit and measures the absorption of infrared light transmitted through the patient's inspired and expired respiratory gases as they flow through the respiratory circuit. Such a mainstream infrared gas analyzer is described in detail by Yelderman et al. in U.S. Pat. Nos. 5,081,998 and 5,095,913 assigned to the present Assignee and hereby incorporated by reference in their entirety. These applications describe an infrared detector and a shutterless optically stabilized capnograph which has no moving parts, which does not require a modulated source of infrared radiation, and which does not suffer from thermal drift. The disclosed infrared detector includes a substantially identical pair of thermopile detectors mounted on the same ceramic substrate and connected in series opposition. Because of this configuration, balanced and equal incident radiation illuminating the pair will produce no signal. Also, because the reference junctions of both detectors are on the same ceramic substrate and at substantially the same temperature, a drift in substrate temperature will produce no discernible change in output signal. In order to make the system respond to incident radiation, an optical filter, or attenuator, with a transmission coefficient of approximately 0.50 is placed over one of the thermopile detectors in the pair. With the filter in place, the system responds to incident radiation but is substantially insensitive to other thermal changes since the effect of a variation in background signals is compensated by subtracting the outputs of the two thermopile detectors. It is desirable to construct a sidestream infrared gas analyzer configuration which implements such infrared detectors. The present invention has been designed for this purpose.
Conventional sidestream analyzers draw a small, continuous sample of the respiratory gases through a fixed sample cell and out through an exhaust port of the sample cell. The analyzer then measures the absorption of infrared light as it is transmitted through the sample cell. Typically, a sidestream analyzer requires a pneumatic sample system which incorporates pumps, tubing and fittings. The sample system may also require valves, flow controls, pressure controls and moisture filters or separation devices. For example, a simple configuration which uses a pump to supply the sample gas to the sample cell is illustrated by Passaro et al. in U.S. Pat. No. 4,692,621. Conventional mainstream infrared gas analyzer configurations, on the other hand, take advantage of the primary flow of the respiratory gases and hence do not require the additional complexity of a pneumatic system such as those used in prior art sidestream infrared gas analyzer configurations. It is thus desired in accordance with the present invention to minimize the complexity of the pneumatic system of the sidestream gas analyzer while also improving efficiency.
A mainstream infrared gas analyzer of the type described in the aforementioned Yelderman et al. patents requires the optical and electronic components to be physically connected to the patient's airway or respiratory circuit. As a result, a mainstream gas analyzer may be subjected to mechanical abuse and temperature variations when in use. A sidestream configuration, on the other hand, allows the optical components to be remotely located from the patient's respiratory circuit so that the optical and electronic components (i.e., the optical bench) can be protected by a fixed, temperature controlled housing. Thus, while a mainstream configuration has the advantage of reduced complexity, sidestream configurations are often desired since they have the advantage of protection from damage and thermal gradients.
Conventional sidestream optical benches use infrared detectors which must be stabilized by mechanical chopping techniques. As just noted, conventional sidestream optical benches also require accurate temperature control of the detector environment to assure stability. For example, such a conventional sidestream infrared gas analyzer is disclosed by McClatchie et al. in U.S. Pat. No. 4,177,381. McClatchie et al. therein describe an infrared gas analyzer which utilizes mechanical choppers and temperature controllers in their measurements. McClatchie et al. also utilize a sample cell which directs the air therein so as to prevent direct impingement of oils, particulate matter and other contaminants onto the infrared transparent windows so as to prevent contamination of the windows. Unfortunately, this system is quite complex and expensive and relatively unreliable because of the numerous mechanical elements. A simpler, more reliable sidestream gas analyzer is desired.
Accordingly, an infrared gas analyzer in a sidestream configuration is desired which includes an optically stabilized detector having no moving parts, requiring no temperature control, consuming little power and having a reduced cost, thereby overcoming the problems of conventional sidestream configurations. The present invention has been designed to meet these needs.